The designation of the oldest mosque in the world requires careful use of definitions, and must be divided into two parts, the oldest in the sense of oldest surviving building, and the oldest in the sense of oldest mosque congregation. Even here, there is the distinction between old mosque buildings that have been in continuous use as mosques, and those that have been converted to other purposes; and between buildings that have been in continuous use as mosques and those that were shuttered for many decades. In terms of congregations, they are distinguished between early established congregations that have been in continuous existence, and early congregations that ceased to exist (wikipedia).